Manganese steel

Description

Background of the invention

[0001] This invention relates to a method of producing austenitic manganese steel. This steel is also known as Hadfield Manganese Steel, named for the inventor Robert Hadfield, British Patent No 200 of 1883. In this patent, the upper limit for manganese was set at 20%; in subsequent studies published in 1886, the upper limit was extended to 21 %. Hadfield also discovered the toughening process ("austenitising") by which the properties of the steel, as cast, could be improved, producing exceptional toughness and work-hardening properties, by heating the casting up to 1050° before quenching: British Patent No. 11833 of 1896 and British Patent No. 5604 of 1902. As to the foregoing, see Introduction in Manganese Steel published 1956 by Oliver and Boyd, Edinburgh and London.

[0002] The author of "Austenitic Manganese Steel" (Metals Handbooks, 8th Edition, 1961) states acceptable properties for this steel may be produced up to at least 20%. We are colleagues of the author, and have been for a number of years, and know that in actual practice over a period of many years he perceived and suggested no advantage in exceeding about 14% manganese, 1.2% carbon. The standard alloy, indeed, is and has been about 12% manganese, 1% carbon for a long time. A rule of thumb in the art is that the nominal or desirable carbon limit is about one-tenth the manganese content in percent by weight.

[0003] One major advantage of the steel is its ability to withstand wear because of its inherent work-hardening character. For this reason, castings subjected to constant abuse such as liners and mantlers for gyratory crushers, railroad crossings, teeth for dipper and dredge buckets, wear plates and the like have been composed of this steel.

[0004] From AU-B-69 656/81, there is known an austenitic manganese-carbon steel comprising 1.35 to 2.00 wt% carbon and more than 14 to 26 wt% manganese, the balance being iron. In the manufacture of said known steel, after casting, the melt is very rapidly advanced to a quenching station where a quenching is conducted just after a skin of solidified metal has formed on the casting. Said rapid quench is effected before the steel alloy cools to the austenite transformation temperature.

[0005] From EP-A1-43 808 there is known an austenitic wear-resistant steel having 16 to 25% managanese, 1.1 to 2.0% carbon, 0.2 to 2.0% silicon, 0.5 to 5.0% chromium, 0.1 to 0.5% titanium, 0.3 to 4.0% molybdenum, with or without the addition of up to 0.5% of one or more of Ce, Sn and carbide forming elements such as vanadium, tungsten, niobium, maximim 5% nickel and maximum 5% copper, the remainder being iron and impurities to a maximum of 0.1% phosphorus and 0.1% sulphur.

[0006] We are also aware of US Patent Nos. 4,130,418 and 4,394,168 which address Hadfield steels of high manganese, high carbon content, which will be discussed below.

Objectives of the invention

[0007] The primary objekt of the invention is to improve certain properties of austenitic manganese steel, and especially those identified with increased wear resistance. A related object is to prolong the life of austenitic manganese steel castings subjected to severe abuse in the field of utility.

[0008] Specifically it is an object of the invention to enable more carbon to be incorporated in the alloy to enhance certain properties which are associated with improved wear resistance and to achieve this by dissolving the higher amount entirely in austenite thereby avoiding the possibility of forming embrittling iron carbides at the grain boundaries. In other words, an object of the invention is to be able to incorporate more carbon in the alloy to improve wear resistance and to do this without risking formation of any consequential carbides at the grain boundaries or elsewhere in the casting. Specifically we achieve this object by the method defined in the only claim.

[0009] We were aware of a harder grade of austenitic manganese steel, harder than the standard grade (12% manganese, 1 % carbon) but also that the same alloy does not perform well in the field, actually breaking up before the expected service life due to brittle failure.

[0010] The documents on this alloy (US Patent Nos. 4,130,418 and 4,394,168) postulate manganese up to 25% and carbon in the range of 1 to 2% (see US Patent 4,394,168) while employing carbide formers such as titanium, with or without chromium (see US 4,130,418). The second patent (4,394,168) recognises and addresses the embrittlement problem at higher carbon levels, recognised by us, and seeks to overcome it by employing molybdenum (itself a strong carbide former) to spherodise carbides to render the alloy more ductile. While molybdenum is capable of serving in this role, it also has the reputation of inducing incipient fusion at the grain boundaries at a temperature below that needed for adequate solution of the carbon and austenite. This would weaken the alloy.

[0011] In the US patents referred to above, the highest level of manganese suggested is 23% (Patent No. 4,130,418) and 24% according to Patent No. 4,394,168. In the actual working examples, however, no values above 22% are given.

[0012] We reasoned that at higher levels of manganese, say 25% by weight or higher, the thermodynamic activity of carbon in austenite is lowered and the nucleation rate of carbide (Fe, Mn)₃C is slower thus aiding supersaturation of carbon in the austenite phase during the water quench following heat treatment (solutionising). The kinetic effect of the higher manganese content would tend to offset the thermodynamic effect of the higher carbide addition, that is, the greater driving force for cabide precipitation. The alloy should therefore show super resistance to gouging abrasion without addition of any strong carbide former, such as chromium, molybdenum and titanium and indeed the highest degree of solubility would be achieved for carbon so that there should be no embrittling carbides (eg. iron-manganese carbides) of any consequence at the grain boundaries or elsewhere in the casting. The results should be a superior alloy with no intentional addition of any carbide former. It should be noted, however, that in melting practice when using scrap steel some chromium might be present in an inconsequential amount and a small amount of aluminium deoxidiser may also be present in our alloy.

Preferred embodiments of the invention and comparisons

[0013] The following test data bear out our conclusion and establish superior work-hardening ability for our alloy when employing enough manganese (e.g. 25%) to dissolve all carbon at levels of 1-4% or higher, rather than coupling carbon to strong carbide forming elements such as chromium, molybdenum or titanium.

[0014] Test casting from these heats were subjected to the standard heat treatment of 1040°C-1095°C (1900°F-2000°F) for one to two hours, depending upon section thickness.

[0015] It is well known in the art that the high work-hardening rates of austenitic manganese steel make it a very suitable choice in many crusher applications. Thus, specimens taken from experimental castings were tested in tension to determine work-hardening rate, that is, the ratio of the increases in stress required to produce successive increments of strain. The steel with superior work hardenabilty will show a greater increment of stress needed to produce the same increment of strain, that is, the slope of the stress-strain curve will be steeper for the superior alloy. The results are given in Table 11.

[0016] Examination of photomicrographs of these steels shows substantially no carbides in the microstructure and certainly no such impairment of this kind at the grain boundaries. Compared to standard Hadfield Manganese Steels, these steels show greater mechanical twin densities after deformation. This results in an increased work-hardening rate in the latter.

[0017] The work-hardening rates for the steels of Table I are to be compared to those in which high manganese and high carbon are coupled to strong carbide formers, intentionally added, such as chromium, molybdenum and titanium, per Tables III and IV following.

[0018] It can be readily seen from these comparisons that addition of strong carbide forming elements to a high manganese, high carbon austenitic manganese steel detracts from work hardenability and doubtless accounts for brittle failure, both reported from field experience and documented as noted above. In comparison the field (actual service) experience in testing our alloy, devoid of strong carbid forming elements, shows outstanding performance especially in gyratory crusher (liner) service.

[0019] The results are corroborated by comparing yield strength and tensile strength for extremely thick sections where high values are traditionally equated to better service life for manganese steel liners in gyratory crushers. Here (Table V) the sections were of identical thickness, 14 cm (5 1/2") and heat-treated to the same parameters, namely 1095°C (2000°F) for two hours (after hot shakeout of the casting) with double end quench in water.

[0020] The chemistry of heat 063 is given in Table I. The chemistry heat for 359 is given in Table III. The alloy without carbide formers exhibits superior strength and work hardening rate.

[0021] We perceive no good reason to exceed a carbon value of about 1.4 to 1.6, nor a manganese value of about 24-28, representing a (weight) two percent allowance on either side of 26%. Increasing amounts of carbon above 1.4% do result in a greater work-hardening rate (Table II) and will be dissolved by 25% manganese (e.g. heat 234, 1.7% carbon) but clearly the optimum is about 1.4 to 1.5% carbon. A satisfactory range for the present alloy is therefore (by weight %)

[0022] A balance iron except for impurities ((e.g. sulphur and phosphor), deoxidisers (e.g. aluminium) and tramp elements (e.g. chromium and nickel) in scrap steel employed in melting practice).

Claims

A method of producing austenitic manganese steel without forming embrittling carbides at the grain boundaries or elsewhere in the casting, and devoid of intentionally added carbide-forming elements, comprising the steps of casting a steel alloy having the following composition in percent by weight, manganese 24-28, carbon 1.4-1.6 and silicon 0.1-1, balance iron except for small amounts of impurities, removing the casting from the mold without any intervening quench and solutionising the casting by heat treatment at 1038-1093°C (1900-2000°F) and finally water quenching the casting, wherein the high manganese content aiding supersaturation of carbon in austenite during the water quench following heat treatment, and the amount of manganese and carbon, and the heat treatment temperature being chosen to produce a work-hardening rate of 1765 N/cm² (256 Ksi), or better, for the casting.

Ansprüche

Verfahren zum Herstellen von austenitischem Manganstahl ohne Ausbildung spröder Carbide in den Korngrenzen oder anderswo beim Gießen und ohne absichtlich zugefügte carbidbildende Elemente, umfassend die Schritte des Abgießens einer Stahllegierung mit der folgenden Zusammensetzung in Gew.- % Mangan 24-28, Kohlenstoff 1,4-1,6 und Silicium 0,1-1, Rest Eisen mit Ausnahme geringer Mengen an Verunreinigungen, Entnehmens des Gußstückes aus der Gußform ohne intervenierende Abkühlung und Lösungsglühens des Gußstücks mittels einer Wärmebehandlung bei 1038-1093°C (1900-2000°F) und eines abschließenden Abschreckens des Gußstückes in Wasser, wobei der hohe Mangangehalt bei der Übersättigung des Austenits mit Kohlenstoff während der der Wärmebehandlung folgenden Abkühlung in Wasser mithilft, und wobei die Menge an Mangan und Kohlenstoff sowie die Temperatur der Wärmebehandlung so gewählt werden, daß eine Verformungsverfestigungsrate von 1765 N/cm² (256 Ksi) oder mehr vom Gußstück erreicht wird.

Revendications

Procédé de fabrication d'un acier au manganèse austénitique sans formation de carbures fragilisants aux limites des grains ou ailleurs dans le moulage, et exempt d'éléments formateurs de carbure ajoutés intentionnellement, comprenant les stades de coulée d'un alliage d'acier ayant la composition suivante en pourcentage pondéral, manganèse 24-28, carbone 1,4-1,6 et silicium 0,1-1, le reste de fer à l'exception de faibles quantités d'impuretés, d'enlèvement du moulage du moule sans aucune trempe intermédiaire et de passage du moulage en solution par traitement thermique à 1038-1093°C (1900-2000°F) et enfin de trempe à l'eau du moulage, dans lequel la teneur élevée en manganèse aide à la sursaturation du carbone dans l'austénite au cours de la trempe à l'eau faisant suite au traitement thermique, et la quantité de manganèse et de carbone et la température de traitement sont choisies pour donner un taux d'écrouissage de 1765 N/cm² (256 Ksi), ou davantage pour le moulage.